This study will evaluate the effects of two different diets on the rate of progression of renal failure in patients with chronic renal insufficiency and glomerular filtration rates between 25 and 60 ml/min/1.73 square meters. The study will be carried out in nine different centers involving 12 hospitals. The control diet will provide an intake of protein (1.0-1.4g/kg/day) and phosphorus (1200-1600 mg/day) which is typically ingested by patients with this level of renal insufficiency. The experimental diet will provide 0.55-0.60 g/kg/day of primarily high biological value protein and 600-800 mg/day of phosphorus. The project will be divided into four sections: Establishment of a definite protocol and writing the procedures manual (9 mos), a pilot study (18 mos), a full scale cooperative clinical trial (FSCCT) (5 yrs), and data analysis and report writing (12 mos). During the FSCCT, each patient will traverse a series of phases. The chief ones include a Baseline Phase where baseline data will be obtained, a Control Phase (up to 5 mos) in which all patients will ingest the control diet and blood pressure will be intensively controlled and monitored, a Maintenance Phase (up to 53 mos) where, after randomization, patients will be treated with either the control or experimental diet and continue careful blood pressure control, and the Termination Phase, where final outcome measurements will be obtained. Both the Control and Maintenance Phases will be preceded by a Training or Induction Phase where patients will be trained in dietary therapy and blood pressure control. The key measures of renal function will be inulin and para-aminohippurate clearances, which will usually be performed every 3-4 months. Creatinine clearances will be evaluated, generally monthly. A key feature is that dietary compliance will be closely monitored by repetitive measurements of urea nitrogen appearance (UNA), as well as dietary interviews and analyses of serum and urine phosphorus, sodium and potassium. In order to maximize dietary compliance, the results of each UNA and phosphorus measurement will be rapidly transmitted to the principal investigator at each center and to the patient. Nutritional status will be evaluated serially by anthropometry, 3H-water and bromide spaces, and serum protein analyses. Patient's attitudes toward dietary therapy and other aspects of treatment, psychosocial and economic status, rehabilitation, hyperparathyroidism, clinical status, intercurrent illnesses and hospitalization will also be monitored. A protocol for the FSCCT is submitted; it will be modified according to the results of the pilot studies.